The primary avalanche concern for the forecast period will be Wind Slabs and Storm Slabs above ~7000’.
Storm Slab – An additional one to three feet of new snow is forecasted for the region above ~8000’ over the next 24 hours, elevating the risk of Storm Slab avalanches for terrain above ~7000’. The rapid loading will produce potentially large destructive avalanches running into the lower elevations. Travel in or under avalanche terrain not recommended. Natural avalanches likely, human triggered avalanches very likely.
Wind Slab – Strong to extreme Southwesterly winds and plenty of transportable snow will result in sensitive Wind Slabs forming primarily on NW-N-NE-E-SE aspects, above ~7000’ in favored locations (crossloaded – gullies and shallow depressions, near and adjacent to rock outcroppings, below ridgelines, etc.) However, localized channeling and turbulent winds can produce Wind Slabs further down slope than anticipated and on unusual aspects, in relatively sheltered terrain, and potentially extending further down slope than usual. Travel in or under avalanche terrain not recommended. Natural avalanches likely, human triggered avalanches very likely.
Likelihood ?CertainVery LikelyLikelyPossible
Size ?HistoricVery LargeLarge
Strong Southwesterly winds and plenty of transportable snow will result in sensitive Wind Slabs forming primarily on NW-N-NE-E-SE aspects, above ~7000. Due to the strong gusty winds or localized wind channeling, Wind Slabs may be encountered on unusual aspects, in normally sheltered areas, as well as further down slope than usual. Wind Slabs will likely be encountered along ridgelines, crossloaded gullies and depressions, in and around terrain features that promote drifting and loading. Be wary of hollow, drum-like sounding snow or cracks shooting out from your feet. Travel in or under avalanche terrain not recommended. Natural avalanches likely, human triggered avalanches very likely.
Likelihood ?CertainVery LikelyLikelyPossible
Size ?HistoricVery LargeLarge
Another Atmospheric River event has rolled into the region overnight and has produced snowfalls of 1 to 2 feet of snow above ~8000 around Mammoth and June (lesser amounts elsewhere) with an additional 1 to 3 feet of snow forecasted thru the next 24 hours over the higher terrain, elevating the potential for wide spread Storm Slab development above ~7000’, where areas recieve or will recieve 12" or greater snowfall. The rapid loading will likely produce potentially large destructive avalanches running into the lower elevations. Travel in or under avalanche terrain not recommended. Natural avalanches likely, human triggered avalanches very likely.
*Possible Loose Dry avalanches possible in steep shelter mid elevations where new snowfall amounts exceed 12". May be small in nature but can be a threat when combined with terrain traps.
Spring had been in full swing in the Sierra for the past week with high pressure, sunny skies, and seasonable to above seasonable temperatures interrupted by the occasional fast moving shortwave passing through the region and depositing a few inches of snow before moving east. The snowpack has mostly consolidated and is primarily characterized by Melt-Freeze crusts separated by rounds or decomposing grains closer to the snow surface with isolated pockets of cold winter like snow on Northerly to Northeasterly aspects in the upper elevations . Thursday (4/6/17) winter made a late season reappearance with the arrival of another Atmospheric River event forecasted to impact the Sierras thru Saturday. Thursday, snow levels initially in the Central Sierra hovered around 9000 feet in Mono County with precipitation picking up in the late afternoon as strong convective showers moved through the region. Overnight, heavy precipitation began impacting the Sierra with a Winter Storm Warning issued for the Sierra tonight through Saturday with storm totals of 2’ to 4’ of new snow forecasted. The sub-tropical nature of the system will produce heavy dense snow, which hasn’t had sufficient time to bond and sinter to the old snow surface and possible density changes within the new snow will likely form sensitive Storm Slabs at all elevations. As a result, the risk of Storm Slab avalanches has increased substantially and will remain a threat for the forecast period. Additionally, the current storm system moved into the region Thursday night with strong to extreme Southwesterly winds (gusts over 100 mph along the Crest) forming widespread sensitive Wind Slabs on NW-N-NE-E-SE aspects, primarily mid to upper elevations. These may be encountered below ridgelines and near and adjacent to terrain features that promote crossloading. Localized channeling and turbulent winds can produce Wind Slabs further downslope than anticipated and on unusual aspects, in relatively sheltered terrain, and potentially extending well into the mid elevations (8000’-10000’). Travel in avalanche terrain not recommended.
24 Hour Precipitation Totals and Temperatures (6:00 AM, 4/7/17)
Location Snow Water Temps
VA Lakes (9445’) N/A 1.2” 30
Tioga Pass (9798’) 7” N/A N/A
Ellery Lake (9645’) N/A .6” 29
June (9148’) 13” .6” 25
Gem Pass (10750’) N/A .9” N/A
Agnew Pass 11” N/A 26
Mammoth Sesame St (9014’) 17” 2” 25
Mammoth Pass (9,500’) N/A N/A 25
Rock Creek (9600’) 8” N/A 27
Saw Mill-Big Pine (10200’) 3” N/A 27
Big Pine Creek (10000’) 5” N/A N/A
...WINTER STORM WARNING IN EFFECT FROM MIDNIGHT TONIGHT TO 5 PM PDT SATURDAY...
- Timing: Heavy wet snow will develop before daybreak Friday in the Sierra and areas west of Highway 395, including Mammoth Lakes. Snow will spread to communities along Highway 395 by Friday night through Saturday. Snow should taper to showers on Saturday.
- Snow Levels: Snow levels starting 7000-8500 feet Friday, falling to all valley floors by Saturday morning. Snow levels may fluctuate around 7500 feet on Friday.
- Snow Accumulations: Potential for 2 to 4 feet of heavy wet snow along the Sierra Crest and Mammoth Mountain, generally above 8500 feet. 10 to 20 inches in Mammoth Lakes and passes on Highway 395. 2 to 5 inches for Lee Vining and Bridgeport, with more possible if snow levels fall faster than expected.
- Winds: Strong southwest winds area wide, with gusts Thursday night through Saturday morning ranging from 45-60 mph along Highway 395, and up to 150 mph along the higher mountain peaks and ridges.
- Impacts: Significant travel disruptions Friday around Mammoth Lakes and at pass elevations along Highway 395 including Conway, Deadman, and Devils Gate summits. Travel disruptions becoming more widespread in Mono County Friday night into Saturday. Heavy wet snow and strong winds could lead to downed trees and power outages.
Friday thru Saturday - heavy precipitation moved into the region overnight (Thursday) with liquid totals averaging 1.5-2.5 inches of accumulation for higher Sierra locations for the first wave of moisture with snow levels around 7000-8000 (+/- 500’) in Mono County. Winds along the Sierra Front, 20-35 mph and gusts to 55 mph for many areas. The one exception looks to be along the Highway 395 corridor with winds of round 40 mph with gusts to 60-65 are likely during the afternoon. Winds will weaken some Friday night as the next wave of precip moves in but will remain breezy with Sierra ridge winds continuing to gust over 100 mph. As the day progresses, the cold-core of the low will move into the region with more moderate precipitation accumulations as compared to yesterday with the region generally in between two stronger waves. Another more intense wave of precipitation will move through later this evening (Friday) through early Saturday. The bulk of the precipitation will be directed from Tahoe Basin southward through Mono County bringing continued heavy snow above 5500 ft in the Sierra and potentially some light accumulations for lower valleys along the Sierra Front. Breezy post-frontal conditions will continue Saturday behind the main cold front as ridge winds diminish. Instability increases Saturday as more of the cold-core portion of low pressure moves into the region with a low chance of a stray thunderstorm.
Sunday into next weekend - It will remain cool Sunday behind the current system with weak ridging expected Monday into Tuesday. Models and ensembles are showing a hybrid winter/spring storm expected late Wednesday into Friday. It will draw in some subtropical moisture as it moves in and snow levels also look colder. Precip may begin Wednesday night and continue off and on through Friday. Snow levels could start 6-7000 feet and possibly drop to the valley floors late.